1. Field of the Invention
The present invention relates to a process for the preparation of mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates, i.e. crude MDI, having a reduced iodine color number by reacting the corresponding mixture of a diphenylmethanediamine and a polyphenylpolymethylenepolyamine, i.e. crude MDA, with phosgene in the presence of one or more inert organic solvents, wherein a low molecular weight alkanol or a polyhydric alcohol or a mixture thereof is incorporated, in an effective amount, into the reaction mixture after the end of the phosgenation.
2. Description of Related Art
Crude MDI, one of the industrially most important starting materials for the production of polyisocyanate polyadducts, for example foams containing urethane groups or urethane and isocyanurate groups, and of diphenylmethane 4,4'-diisocyanate, an important component for the production of polyurethane (PU) elastomers, fibers, sealing compounds, adhesives, etc., is known to be prepared by phosgenating crude MDA, usually in the presence of an inert organic solvent. Crude MDA in turn is obtained by condensation of aniline and formaldehyde in the presence of acidic catalysts, and the percentage of diphenylmethanediamines and of the homologous polyphenylpolymethylenepolyamines and their isomers can be controlled as a function of the selected ratios of the starting materials and the reaction conditions and the various methods (Kunststoff-Handbuch, Volume 7, Polyurethane, 1st edition 1966 and 2nd edition 1983, Carl-Hanser-Verlag, Munich, Vienna). If the condensation of aniline and formaldehyde is carried out, for example, in the presence of weakly acidic catalysts, crude MDA mixtures having a relatively high content of 2,2'-and 2,4'-diaminodiphenylmethanes are obtained, while crude MDA mixtures having a high content of 4,4'-diaminodiphenylmethane and at the same time a low content of 2,4'-diaminodiphenylmethane can be prepared only in the presence of relatively large amounts of strongly acidic catalysts, preferably of strong mineral acids, e.g. hydrochloric acid.
The ratio of diaminodiphenylmethane isomers to the higher homologs in the crude MDA is furthermore dependent on the aniline/formaldehyde ratio and on the condensation temperature, higher aniline/formaldehyde ratios and low condensation temperatures resulting in high diaminodiphenylmethane contents (CA-A-700 026).
The disadvantage of these preparation processes, which are described in many publications in the literature and in many patents, is the formation of more or less strongly colored crude MDA, whose color may vary from black through darker and paler brown hues to ochre. Another disadvantage is that these discolorations are reduced insufficiently, if at all by the subsequent phosgenation for the preparation of the corresponding crude MDI, and the crude MDI formed cannot be purified by distillation. This undesirable discoloration is further-more effective in the secondary products, so that even the noncellular or cellular polyisocyanate polyadducts produced from colored crude MDI are not colorless. Although the intrinsic color of the polyisocyanate polyadducts does not have an adverse effect on their mechanical properties, the consumer wants essentially colorless products.
There has therefore been no lack of attempts to reduce the discoloration of crude MDI and to stabilize the prepared polyisocyanates by suitable process measures or additives.
According to U.S. Pat. No. 2 885 420, organic polyisocyanates can be stabilized to discoloration by the addition of from 0.01 to 0.5% by weight, based on the weight of polyisocyanate, of an aromatic, cycloaliphatic or aliphatic ether or thioether.
To eliminate impurities in organic diisocyanate solutions, which impurities act as gelling catalysts, according to DE-A-1 280 855 (GB I 097 219) about 0.001-1% by weight, based on the weight of the diisocyanate, of phosphoric acid is added to said solutions.
GB-B-1 465 014 describes the addition of glycidol in an amount of from 0.001 to 0.25% by weight, based on the weight of diisocyanate, for improving the shelf life of distilled diphenylmethane diisocyanates.
EP-B-0 183 976 (U.S. Pat. No. 4,677,221) relates to a process for the preparation of (cyclo)aliphatic diisocyanates having high-temperature color stability, wherein a technical-grade diisocyanate having aliphatically and/or cycloaliphatically bonded isocyanate groups is heated in the presence of from 0.1 to 3% by weight of a compound which is soluble in the diisocyanate and has not less than 3% by weight of structural units of the formula --NH--CO-- for not more than 5 hours at from 100.degree. to 220.degree. C., and the diisocyanate treated in this manner is then purified by distillation. The process is not applicable to the treatment of crude MDI, since, as stated above, the latter cannot be distilled.
According to U.S. Pat. No. 4,465,639, from 0.1 to 5% by weight, based on the weight of polyisocyanate in the reaction mixture, of water are incorporated into crude MDI after the end of phosgenation but before the phosgene has been completely separated off. This measure makes it possible to lighten the color of the crude MDI and of the PU foams produced therefrom. Furthermore, the proportion of relatively high molecular weight MDI homologs in the crude MDI is substantially decreased and their viscosity reduced. Although this makes it possible to reduce the iodine color number of the crude MDI, this method also has considerable disadvantages. As a result of the presence of water, the corrosive effect of the reaction mixture containing chlorine, hydrogen chloride and phosgene on the apparatuses of the production plant is substantially increased and therefore the risk of leakage, associated with the escape of toxic phosgene or of a phosgene-containing reaction mixture, is increased. For safety reasons, moisture in any form is therefore advantageously essentially completely excluded during phosgenation.